1. Field of the Invention
The present invention relates in general to an image formation optical system and, an image reading apparatus using the same. In particular, the invention is suitable for an image reading apparatus such as an image scanner, a copying machine or a facsimile which is adapted to sufficiently exhibit the optical performance of an image formation optical system having a rotation asymmetric lens such as an anamorphic lens.
2. Related Background Art
FIG. 20 is a schematic view showing a construction of a main portion of a conventional image reading apparatus.
In the figure, reference numeral 92 designates an original table glass on the surface of which an original 91 is placed. Reference numeral 97 designates a carriage in which an illumination system 93, a plurality of reflecting mirrors 94a, 94b, 94c, 94d and 94e, an image formation optical system (image reading lens) 95, a reading unit 96, and the like all of which will be described later are integrally accommodated. Image information of the original 91 is read by scanning the carriage 97 in a sub-scanning direction indicated by an arrow in the figure by a sub-scanning mechanism 98 such as a motor. The image information which has been read is sent to a personal computer or the like as an external apparatus through an interface (not shown).
Reference numeral 93 designates an illumination system composed of a xenon tube, a halogen lamp, an LED array or the like. By the way, a reflecting plate such as an aluminium evaporation plate may be combined with the illumination system 93. Reference numerals 94a, 94b, 94c, 94d and 94e designate reflecting mirrors, respectively, which serve to fold optically the luminous flux from the original 91 in the inside of the carriage 97. Reference numeral 95 designates an image formation optical system which serves to image the light from the original 91 on the surface of the reading unit 96. Reference numeral 96 designates a linear sensor such as a CCD (Charge Coupled Device) as the reading unit which is constructed by arranging a plurality of light receiving elements in a main scanning direction as a direction perpendicular to the paper surface.
For miniaturization of the image scanner in the above-mentioned construction, the carriage 97 must be miniaturized. For miniaturization of the carriage 97, for example, there is a method of increasing the number of reflecting mirrors, or a method of reflecting the light plural times by one reflecting mirror to ensure an optical path length.
However, in these methods, there is encountered a problem in that since the internal construction of the carriage 97 becomes complicated, it becomes difficult to achieve accurate assembly and hence the cost is greatly increased. In addition, there is also encountered a problem in that the image formation performance becomes worse in proportion to the profile irregularity of the reflecting mirror and the number of times of reflection, which exerts an influence on the read image as well.
Then, the applicant of the present invention made such a proposal that in the formerly proposed image reading apparatus shown in the Japanese Patent Application Laid-Open No. 2000-171705, an anamorphic lens having at least one surface which is rotation asymmetric with respect to an optical axis is introduced into an image formation optical system to increase the field angle of the image formation optical system (image formation system) 97 to shorten the distance between object and image, thereby shortening the optical path length itself.
However, if the lens having the anamorphic surface is incorporated in the image formation optical system, then the image formation performance of the image formation optical system will become rotation asymmetric with respect to an optical axis. For this reason, the main scanning direction of the image formation optical system, and the direction of arrangement of a plurality of light receiving elements constituting a CCD of the image reading apparatus need to be controlled so as to be aligned with each other.
Various image reading apparatuses for fixing accurately the above-mentioned image formation optical system in the main scanning direction along which a CCD is arranged have been proposed Application Laid-Open No. 2000-307823 and Japanese Patent Registration No. 307,828 for example. In accordance with these references, the positional relationship among an anamorphic lens, a lens barrel and an image reading apparatus is accurately fixed.
In addition, there has been proposed, e.g., in Japanese Patent Application laid-Open No. 2002-314765, a method of maintaining the high image formation performance of the above-mentioned image formation system even under the conditions in which the temperature environment is severe.
However, these techniques described in these references have the following problems.
First of all, the description will hereinbelow be given with respect to a conventional image formation optical system 101 constituted only by rotation symmetric lenses with reference to FIG. 21. If rotation symmetric lenses a, b, c and d included in a lens barrel 100 are manufactured as designed, as shown in a shaded dot portion in the figure, the sufficient image formation performance is exhibited in the whole image formation area 105. However, as shown in the rotation symmetric lens c in the figure, in general, in the image formation optical system, the lens decentering occurs through the manufacture process. If the lens decentering occurs, then the image formation performance within the image formation surface becomes nonuniform. For this reason, in the image formation optical system 101 constituted by only the lenses each having the rotation symmetric shape, the lenses each having the rotation symmetric shape needs to be rotated with the optical axis as a center to carry out the adjustment indicated by an arrow in the figure (hereinafter, referred to as xe2x80x9cthe rotation adjustmentxe2x80x9d) so that the best image formation line 104 having high image formation performance overlaps the direction of arrangement of a plurality of pixels of the CCD within the image formation area 105 (hereinafter, referred to as xe2x80x9cthe CCD arrangement directionxe2x80x9d).
Next, the description will hereinbelow be given with respect to a prior art using the rotation asymmetric lenses such as the anamorphic lenses with reference to FIG. 22.
In the figure, objects shown in the form of circles are lenses each having a rotation symmetric shape (rotation symmetric lenses) a, b, c and d, and an object shown in the form of a quadrilateral is a lens having a rotation asymmetric shape (rotation asymmetric lens) e. The main scanning direction of the rotation asymmetric lens e is indicated by a long broken line x in the quadrilateral. In the image formation optical system 101 using the rotation asymmetric lens e, the image formation area 105 becomes a flattening area determined by the rotation asymmetric lens e.
In this image formation optical system 101, when the lens decentering as exemplified by the rotation symmetric lens c occurs, the sufficient image formation performance can not be obtained. Even if the rotation adjustment is carried out, since both the best image formation line 104 and the image formation area 105 are rotated, there is a problem in that the CCD arrangement direction 106 and the best image formation line 104 are not necessarily aligned with each other.
Thus, for the image formation optical system 101 using the rotation asymmetric lens e, the molding accuracy, the assembly accuracy and the like of the lens barrel need to be rigidly managed so that a quantity of occurrence of the decentering becomes very small. Thus, that image formation optical system 101 can not be safely said as the image formation optical system which is excellent in mass production due to an increase in processing cost and a reduction in yield.
In addition, the above-mentioned problem is also the problem arising similarly due to the fact that even in the case of a lens having no anamorphic surface, if the lens concerned is the so-called flattening lens having the rotation asymmetric shape in outer diameter, then the image formation surface is flattened.
When the positional relationship between the anamorphic lens and the barrel is controlled, in actuality, there is a limit to the assembly accuracy, and hence the values provided through the assembly are difficult to perfectly coincide with the design values. Thus, a measure of an error occurs. In addition, with respect to the positional relationship as well between the barrel and the image reading apparatus, likewise, the mounting error occurs.
Thus, at least two errors occur in the alignment of the main scanning direction of the anamorphic lens and the main scanning direction of the image reading apparatus, and a quantity of error becomes unignorable in the field of the image reading apparatus requiring the high accuracy.
FIG. 23 is a cross sectional view of an image formation optical system OL using the anamorphic lens described in the Japanese Patent Application Laid-Open No. 2000-171705. FIG. 24 is a view useful in explaining the various aberrations in the image formation optical system OL. A surface R10 of the lens of the lenses shown in FIG. 23 is an anamorphic surface. For the anamorphic surface R10, an angle defined between a normal and an optical axis in the most outer peripheral portion through which a beam of light passes, i.e., the so-called opening angle reaches no less than 45xc2x0.
In the case where an anamorphic lens is made through the cutting processing using a cutting tool or the like, the frictional force applied to the cutting tool is further increased as the opening angle is increased, and hence the highly accurate processing can not be carried out. Also, in the case where an anamorphic lens is made through the glass mold or the plastic molding using a metallic mold, similarly to the foregoing, there is also a problem in that not only the processing accuracy of the metallic mold is poor but also the shape of the metallic mold can not be accurately transferred to the moldings.
In the measurement as well of the lens shape, a large frictional force is caused between a measuring probe and a lens surface, and hence not only the accurate measurement can not be carried out, but also the measuring probe may be damaged in some cases.
While the design for decreasing an opening angle has been attempted in order to solve the above-mentioned problems, correction for distortion aberration, coma aberration and the like can not be sufficiently carried out and hence it is impossible to maintain the high image formation performance.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore a first object of the present invention to provide an image formation optical system which is excellent in mass production due to a reduction in processing cost and an improvement in yield, and an image reading apparatus using the same.
It is a second object of the present invention to provide an image reading apparatus which is capable of exhibiting sufficiently the optical performance of an image formation optical system at least one surface of which has an anamorphic surface to read image information with high accuracy.
It is a third object of the present invention to provide an image formation optical system which is excellent in image formation performance while reducing an opening angle of an anamorphic lens, and an image reading apparatus using the same.
In order to attain the above-mentioned objects, according to one aspect of the present invention, an image formation optical system for imaging image information of an original on a photoelectric conversion device having a plurality of pixels arranged in onexe2x80x94dimensional direction is characterized in that a lens barrel for holding at least one of rotation symmetric lens having a shape which is rotation symmetric with respect to an optical axis, and a lens barrel for holding a rotation asymmetric lens having a rotation asymmetric shape are assembled so as to be mutually rotatable for adjustment.
According to another further aspect of the present invention, an image formation optical system for imaging image information of an original on a photoelectric conversion device having a plurality of pixels arranged in one-dimensional direction includes: at least one of rotation symmetric lens having a shape which is rotation symmetric with respect to an optical axis and held inside a lens barrel; and a rotation asymmetric lens having a rotation asymmetric shape and held outside the lens barrel, and in the image formation optical system, the at least one of rotation symmetric lens and the rotation asymmetric lens are assembled so as to be mutually rotatable for adjustment.
Incidentally, according to the above-mentioned image formation optical system, it is preferable that the rotation asymmetric lens has a flattening shape in outer diameter.
Further, according to the above-mentioned image formation optical system, it is preferable that the rotation asymmetric lens has an anamorphic surface in optical surface shape.
According to another further aspect of the present invention, an image reading apparatus for imaging image information of an original on a photoelectric conversion device having a plurality of pixels arranged in one-dimensional direction using the above-mentioned image formation optical system to read the image information is characterized in that one direction of a refracting power of a rotation asymmetric lens of the image formation optical system is aligned with the direction of arrangement of the plurality of pixels of the photoelectric conversion device.
According to another further aspect of the present invention, an image reading apparatus includes: an image formation optical system for imaging image information of an original; a barrel for holding the image formation optical system; barrel supporting means for supporting the barrel; and reading means arranged in an image formation position of the image formation optical system and having a plurality of light receiving elements arranged in one-dimensional direction, and in the image reading apparatus, the image formation optical system has at least one of rotation asymmetric lens having a rotation asymmetric shape, and a second alignment reference surface provided in the rotation asymmetric lens is brought into contact with a first alignment reference surface provided in the barrel supporting means to align certain one direction of a refracting power of the rotation asymmetric lens with the direction of arrangement of the plurality of light receiving elements.
Incidentally, according to the above-mentioned image reading apparatus, it is preferable that the second alignment reference surface is a flat surface portion provided in a part of the outer peripheral portion of the rotation asymmetric lens, and the first alignment reference surface is a flat surface portion provided in a part of the barrel supporting means.
Further, according to the above-mentioned image reading apparatus, it is preferable that one of the alignment reference surfaces is a protruding portion, and the other of the alignment reference surfaces is a groove portion adapted to be engaged with the protruding portion.
Further, according to the above-mentioned image reading apparatus, it is preferable that a fourth alignment reference surface provided in the rotation asymmetric lens is brought into contact with a third alignment reference surface provided in the barrel supporting means to align a direction perpendicular to the certain one direction of the refracting power of the rotation asymmetric lens with the direction of arrangement of the plurality of light receiving elements.
According to still another further aspect of the present invention, an image formation optical system for imaging image information of an original on a photoelectric conversion device having a plurality of pixels arranged in one-dimensional direction is characterized in that the image formation optical system has an aperture stop, at least one of lens of a plurality of lenses constituting the image formation optical system is an anamorphic lens, and a meniscus lens is arranged adjacent to the anamorphic lens, and when a thickness of the anamorphic lens is da, a thickness of the meniscus lens is dm, an interval between the anamorphic lens and the meniscus lens adjacent thereto is tin, and a distance between a lens surface of the lens of both the lenses nearest the aperture stop and a lens surface of the lens, other than both the lenses, facing the lens surface of the lens of both the lenses nearest the aperture stop is tout, the following condition is satisfied:
tin less than dm+da less than tout. 
Further, according to the above-mentioned image formation optical system, it is preferable that a maximum opening angle in a ray passing area of the anamorphic lens is equal to or smaller than 40xc2x0.
Further, according to the above-mentioned image formation optical system, it is preferable that when a focal length of a main scanning cross section of the image formation optical system is fall, and a maximum outer diameter of a ray passing area of the anamorphic lens is K, the following relationship is established:
0.3xe2x89xa6K/fallxe2x89xa61.5. 
Further, according to the above-mentioned image formation optical system, it is preferable that the meniscus lens adjacent to the anamorphic lens is a meniscus-shaped lens having a negative refracting power and having a recess surface directed to the aperture stop side.
Further, according to the above-mentioned image formation optical system, it is preferable that a refracting power within a main scanning cross section of an anamorphic surface of the anamorphic lens, and a refracting power within a sub-scanning cross section thereof are changed within an effective range, and when a differences between maximum values of the refractive power and minimum values of the refractive power are xcex94"PHgr"m and xcex94"psgr"s, respectively, the following condition is satisfied:
0.008 less than xcex94"PHgr"m less than 0.045 and 0.000 less than xcex94"PHgr"s less than 0.012. 
Further, according to the above-mentioned image formation optical system, it is preferable that when a focal length of a main scanning cross section of the image formation optical system is fall, a focal length of the meniscus lens is fm, and a focal length of a main scanning cross section of the anamorphic lens is fa, the following condition is satisfied:       -    1.65     less than             f      m              f      all         less than             -      1.20        ⁢          xe2x80x83        ⁢    and    ⁢          xe2x80x83        ⁢    4.0     less than       "LeftBracketingBar"                  f        a                    f        m              "RightBracketingBar"     less than   13.5
Further, according to the above-mentioned image formation optical system, it is preferable that the image formation optical system includes: a meniscus-shaped first lens having a positive refracting power and having a convex surface directed to an original side; a second lens having recess surfaces as both lens surfaces; a third lens having convex surfaces as both lens surfaces; a meniscus-shaped fourth lens having a negative refracting power and having a recess surface directed to the original side; a meniscus-shaped fifth lens having a recess surface directed to the original side; and a meniscus-shaped sixth lens having a recess surface directed to the original side in this order from the original side, and that the anamorphic lens is at least one of lens of a plurality of meniscus-shaped lenses.
Further, according to the above-mentioned image formation optical system, it is preferable that the image formation optical system includes: a meniscus-shaped first lens having a positive refracting power and having a convex surface directed to an original side; a second lens having recess surfaces as both lens surfaces; a third lens having convex surfaces as both lens surfaces; a meniscus-shaped fourth lens having a recess surface directed to the original side; and a meniscus-shaped fifth lens having a recess surface directed to the original side, and that the anamorphic lens is at least one of lens of a plurality of meniscus-shaped lenses.
Further, according to the above-mentioned image formation optical system, it is preferable that the anamorphic lens is made of plastic mold, and the meniscus lens is made of glass.
According to still another further aspect of the present invention, an image reading apparatus is characterized by forming image information on a surface of reading means using the above-mentioned image formation optical system.